I.

PAPERS OF THE PHYSICAL CLASS.

I. Of certain NATURAL APPEARANCES of the Ground on the Hill of Arthur's Seat. By JAMES HUTTON, M. D. F. R. S. EDIN. and Member of the Royal Academy of Agriculture at PARIS *.

IN

N fummer 1776, Profeffor FERGUSON obferved a particular appearance on the hill of Arthur's Seat, near the fummit, which drew his attention, and which he could not underftand. He then carried Dr BLACK and me to the place, where we found fomething which, at a distance, resembled the withered grass of a foot-path, but which traversed a shoulder of the hill, in fuch a direction as correfponded to neither fheeptrack nor foot-path. Upon a near inspection, it appeared to be a narrow stripe of the grafs quite dead and withered. The breadth of this stripe was about nine, or, in some places, twelve inches; the fides of this track were perfectly defined, without any gradation from green to withered grafs, all the plants in the track being killed, without the contiguous part having fuffered in the leaft. The length of this track was confiderable, a hundred yards or two, extending from the fouth-eaft fide of the

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* This Paper was read before the Philofophical Society of Edinburgh in June 1778. It is now printed by order of the Committee for publication of the Transactions of the Royal Society of Edinburgh.

the fouthmost hill through a hollow, and afcending obliquely the fhoulder of the fummit of Arthur's Seat on the fouth-eaft fide.

At first thunder fuggested itself as having been the cause of this remarkable appearance; but the more we enquired into the particulars of this phenomenon, the greater difficulties occurred with regard to the proper correfpondence of that conjectured cause, as well as for affigning any other with the least degree of probability. It is with a view to make this appear, that the following history is made of the particulars which were at that time, and have been fince obferved.

OBSERVATION 1. THE appearance now defcribed was not the only one of the kind; for, upon examination, I found fimilar tracks, though of various extent, in all the different aspects and fituations, from the fouth fide of the fummit to the north fide of the hill half way down to the plain; but none at the bottom.

OBS. 2. THESE appearances, though recent, or of that year's production, had not been the first thing of that kind which had appeared on the hill; for, parallel to each of thofe tracks of withered grass, there was another perfectly fimilar, which then appeared to us as if it had been made the year before, and was then black, the grass having rotted. The distance of this old track from the new, was, in general, only a few inches, fometimes exceeding near, but rarely or never contiguous.

OBS. 3. THE tracks, now under confideration, have been confidered as a thing continuous in its length; but this it is only in general, or in certain portions where it is fo fometimes for a confiderable extent. In other places, again, it is composed of feveral portions of various lengths, the grass being unaffected betwixt those portions which make up the track; so that, in fome places, the track is made as it were by fpots; and these fpots, although in general longer in the direction of the track, are not always fo, there being in fome places, generally at the

extremity

extremity of the track, fpots whofe length do not exceed their breadth.

OBS. 4. THE regularity with which those two tracks run parallel and near to each other, is not more wonderful than is the correfpondency that is in general to be observed with regard to the construction of these, as confifting either of a continuous track or of separate pieces; and to fo great a degree is carried this resemblance of the two tracks, that, where it is by spots the tracks are made, there the fimilarity, even of the spots, were fometimes remarked, fo that it seemed as if the one had been a copy of the other.

OBS. 5. BESIDES the brown colour of thofe new made tracks, which might be seen at a confiderable distance, (two or three hundred feet), there was another ftripe of a dark green, which might be seen at a still greater distance. Upon more close examination, this last appearance was found to take its origin in some grafs of a very dark green, which, in fome places here and there of the last year's track, began to grow in the black ground and among the rotten grafs; but the greatest part of this deep green was behind the last year's track, and was evidently owing to a fimilar growth of graffes in places which had been formerly killed or withered, and were now almost covered with new plants, which gave a deeper shade of green than the rest of the hill.

THIS laft obfervation led to another; for here a question naturally occurred, That, fince this fucceffion of things had certainly taken place at least three years, how many fucceffive tracks might be detected from the examination of those appearances? With this view I confidered attentively fome places where the marks were most distinct, and could plainly count five or fix fucceffions; the number cannot be accurately afcertained, because those which have been made above three or four years are much effaced, although the colour, and fome other marks, evidently prove, that there had been several more.

OBS.

OBS. 6. THE tracks which have been now defcribed, are not straight lines, but have all more or less of a regular circular nature in them; that is to fay, they are fegments of circular figures, and only approach to the appearance of right lines, in proportion as the figure of which they are the fegments is large, or the fegment small; and in those respects there appears to be great variety. There is, however, one appearance which, at first fight, might impose upon an obfervator, and destroy the generality of this obfervation. It is an inftance or two that occur of a continued line in those tracks; but, in this cafe, the line appears to be made up of feveral fegments, each of which ought to be confidered by itfelf; confequently, here will be acknowledged the operation of the fame general principle by which, in those appearances, a regular figure is produced, and that this figure is in its nature circular.

OBS. 7. THE production of those tracks being fucceffive in. its nature, or operating in different places at different periods of time, fuggests another fübject of enquiry, viz. How far any regularity, or a certain order, may be observed alfo with regard to this operation, as well as with regard to that by which the figure is produced? And this, from obfervation, I think, is determined in the affirmative, fo far as, from all the obfervations I have made, this progrefs feems always to have proceeded in the direction of a line, drawn from the centre, bifecting the fegment; that is to fay, thofe portions of concentric circles are never infcribed, but always circumfcribed; and, for this reafon, it will appear, that thofe circles, of which fegments are exhibited to our observation, must be increasing, and not diminishing, in their diameter.

HAVING thus given an account of what was concluded from the first season of thofe obfervations, before proceeding to give the continuation of their hiftory, it may be proper to obferve, that an unfuccefsful attempt was once made to investigate the

caufe,

caufe, by the infpection of the turf cut up, and compared with that immediately contiguous to the track; for, on that occafion, nothing was found that could give any light into the nature of the operation.

FROM the narration of appearances already made, the history of what has happened fince that fummer, 1776, will be extremely fhort, and may be comprehended in two or three words.

IN the fummer 1776, there was prognofticated a fucceffion of appearances fimilar to those which, from the obfervations then made, had been concluded as having already come to pafs, and been transacted with a certain regularity in a former period of time. The event has fully justified the judgment which was formed at that time, refpecting the order and regularity of the appearances, and has alfo left us in the fame ftate of uncertainty, or rather ignorance, with regard to the cause.

IN the fpring, about the month of April, the grass begins gradually to wither and decay. It is perfectly dead in a little time, that is, in a week or two, and then appears white or withered. Thus, every plant being killed in the new track, thofe vegetable bodies, exposed to heat and moisture, gradually decay, fo as next year to exhibit a dark or black, instead of a light or white track, which it had been the year before; but during the fecond year, the dead plants are still obferved in the turf, which, as it begins to get new plants, lofes gradually the appearance of the old ones, until at last little more can be obferved, than a broad fhade of a much deeper green, which, on the one fide, is compared with the natural verdure into which it fometimes seems gradually to terminate; whereas, on the other fide, the deep green colour of the ground formerly tracked, is contrasted with the yellow or light colour of the withered grafs.

FROM the inspection of the ground, and the history of what has been obferved to happen, nothing is more evident than that this regular fucceffive operation has been now repeated, at least

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